Moisture and Mold problem

I just installed a Venmar vent system and am still getting moisture in the same place on all the corners of the extirior walla where they meet the ceilings. I have replaced drywall in places and added insulation to ceiling. there is plastic between my drywall and trusses because that is building code where I live. I have replaced all my windows and doors also. The roof is only a 2/12 and is ventilated with two wind fans aproximately half way between peak and bottom of roof. I replaced shingles this last year. Any suggestions would be VERY much appreciated. I am going broke and probably causeing more problems trying to fix this moisture problem.